This invention relates to a stainless steel for use in engine gaskets and to a method for its manufacture, and particularly to a stainless steel for manufacturing an engine gasket which has excellent fatigue strength and excellent ability to maintain the shape of a bead potion when a stress load is applied for a long period and to a method for its manufacture.
The present invention also relates to a gasket which is obtained in that manner.
In the past, asbestos and the like have been used as gasket materials for use in apparatuses which increase in temperature, such as engines for use in automobiles or in boats. Recently, in response to increases in engine performance and moves to regulate the use of asbestos by laws, gaskets made of metal, i.e., metal gaskets, have come to be used.
Metal gaskets for use in engines must have various properties necessary for maintaining airtightness of joining surfaces. For example, metal gaskets used in the engines of automobiles, motorcycles, etc. must have properties enabling them to withstand the characteristic varying stresses of engines which are repeatedly applied in an atmosphere of combustion gas.
From the standpoint of sealing materials having similar uses, metal packings are increasingly used in place of O-rings wrapped in asbestos in response to moves to restrict the use of asbestos by laws as described above. In this case, a strip-shaped metal coil is wrapped into the shape of a cylinder, and it is formed into a donut-shaped O-ring to obtain a metal packing.
In the past, SUS301 (AISI301) type steel, which is a work hardened metastable austenite stainless steel which is given a high strength by simply performing cold working, was used as a raw material for these metal gaskets, metal packings, and the like.
For metal gaskets, a sheet having a thickness on the order of 0.1-0.4 millimeters is used as a raw material. In the case of a gasket for use in an engine head, for example, a bead is formed in the periphery of the combustion chamber and in the periphery of water supply holes and oil supply holes, and gas, water, and oil are sealed by the high pressure which is generated when clamping down of the bead is performed. For metal packings, a strip-shaped coil is wrapped into the shape of a cylinder, it is formed into the shape of a donut to form an O-ring, and it is used to maintain the airtightness of a joining surface.
Below, in this specification, such metal gaskets and metal packings will be generically referred to for convenience as xe2x80x9cgasketsxe2x80x9d or xe2x80x9cengine gasketsxe2x80x9d, and stainless steels used therein will be referred to as xe2x80x9cstainless steels for engine gasketsxe2x80x9d.
Even in the past, there were disclosures concerning gasket materials for use in engines in, for example, Japanese Published Unexamined Patent Application No. HEI 4-214841, Japanese Published Unexamined Patent Application No. HEI 5-279802, and Japanese Published Unexamined Patent Application No. HEI 5-117813.
The stainless steels for engine gaskets disclosed in these official publications are all ones which are imparted with prescribed properties of fine, uniform recrystallized grains having an average grain diameter of at most 10 micrometers by performing finishing intermediate rolling at a reduction ratio of at least 50% followed by low temperature, short duration finish annealing.
Namely, this existing technology relates to methods of manufacturing a stainless steel having excellent formability and fatigue properties characterized by using an austenitic stainless steel having components corresponding to SUS301, and by grain refining thereof by causing recrystallization by carrying out annealing at as low a temperature as possible.
However, at present, the properties of engines are continuously improving, and simultaneous with the trend towards higher engine output, the level of properties required of gasket materials has been increasing. However, there are various problems such as that it is difficult to obtain a material which has a fatigue strength which can adequately withstand this trend towards high engine output, that the final product has an inadequate hardness when a low carbon content is used, and that the shape maintaining ability of the portion which is subjected to working to form a bead when stress is applied for long periods (referred to below as resistance to settlement) is inadequate.
An object of this invention is to provide a stainless steel which is suitable for gaskets used in today""s increasingly high performance engines and to a method for its manufacture.
Another object of this invention is to provide an engine gasket which can exhibit that type of excellent performance.
A more specific object of the present invention is to provide a stainless steel for engine gaskets which does not use materials requiring special components but which uses SUS301L stainless steel (roughly corresponding to low carbon AISI301) made of ordinary components and which has properties superior to those of existing materials, i.e., a high fatigue strength and excellent resistance to settlement, and to a method for its manufacture.
For example, a metal gasket used in an automotive engine or the like is subjected to working to form a bead. It is then mounted on an engine block, and it is repeatedly subjected to stress accompanying engine operation (explosions within the cylinders). Therefore, it is required to have adequate fatigue strength to resist this, and it is required to maintain the shape of the bead under such varying stress and to maintain a gas seal, i.e., it is required to have resistance to settlement.
An example of a steel which can cope with such conditions is a stainless steel corresponding to SUS301. As described above, such steels are generally used at present. Among the problems that were seen with such existing technology are the following:
1. In the case of a high carbon steel such as SUS301 (C: at most 0.15%), it is relatively easy to obtain a high hardness and to improve resistance to settlement. However, the more the hardness is increased, the more the fatigue strength ends up decreasing when working to obtain a bead is performed in order to prepare an engine gasket, so it is difficult to obtain both fatigue strength and resistance to settlement. In addition, as a problem during the manufacturing stage, there is the possibility of carbides precipitating due to annealing, and there is a fear of a deterioration of corrosion resistance.
2. In the case of a low C content such as C: at most 0.03%, the corrosion resistance is excellent, and it is possible to increase the fatigue strength to a certain extent, but it is difficult to obtain an adequate hardness. For this reason, it is difficult to obtain an adequate resistance to settlement, and there is fear of a decrease in gas sealing properties.
3. Even higher fatigue strength and resistance to settlement are demanded due to increases in engine power, but with the existing technology using SUS301 type steel, it is difficult to achieve both at the same time, and at present, an increase in performance above existing levels is difficult.
The present inventors found that a sufficient level of hardness can be achieved even for a low carbon steel when temper rolling is carried out on a steel having a mixed structure of a recrystallized structure and a recovered, unrecrystallized structure, or a recovered, unrecrystallized structure corresponding to the structure before recrystallization occurs, which structure is prepared by finish annealing prior to temper rolling in order to reduce the influence of prior working. They also found that due to the remaining effects of previous working, if the strain due to working which is applied to the material after temper rolling has the same working rate as in an existing method, the deformation applied to crystal grains can be made large, and the effect of crystal grain boundaries in the structure on fatigue strength can be decreased. They additionally found that due to these synergistic effects, the fatigue strength can be markedly increased compared to that of existing materials.
The present invention is a stainless steel for engine gaskets characterized by comprising a temper rolled metal structure in the form of a recovered unrecrystallized structure or a mixed structure of a recovered unrecrystallized structure and a recrystallized structure. Namely, a stainless steel for engine gaskets according to the present invention comprises a martensite-containing structure obtained by temper rolling of a recovered unrecrystallized structure, or of a mixed structure of a recovered unrecrystallized structure and a recrystallized structure, both being obtained by annealing.
In this manner, a stainless steel for engine gaskets according to the present invention is one derived from a recovered unrecrystallized structure or a mixed structure of a recovered unrecrystallized structure and a recrystallized structure, both of which are obtained by finish annealing. The crystal structure of the metal structure at this time has a half-value width for the X-ray diffraction measured using CuK xcex1 radiations for (220), (311) planes on austenite matrix of at least 0.15xc2x0 and at most 0.35xc2x0.
From another standpoint, the present invention is a method of manufacturing a stainless steel plate for engine gaskets, in which cold rolling and annealing are repeatedly performed after hot rolling, and then temper rolling is performed, characterized in that the reduction during cold rolling carried out before finish annealing is at least 40%, the subsequently performed finish annealing is carried out in a temperature range of at least 700xc2x0 C. and at most 800xc2x0 C., and the metal structure is made a recovered unrecrystallized structure.
At this time, by carrying out finish annealing in a temperature range of at least 700xc2x0 C. and at most 900xc2x0 C., the metal structure can be made a recovered unrecrystallized structure or a mixed structure of a recovered unrecrystallized structure and a recrystallized structure.
By making the reduction in the temper rolling after finish annealing at least 40%, the formation of martensite can be promoted.
Steels which are the object of the present invention are austenitic stainless steels, and particularly steels corresponding to SUS301(AISI301), and preferably, they are steels containing, by weight percent,
C: at most 0.03%, Si: at most 1.0%, Mn: at most 2.0%, Cr: at least 16.0% and at most 18.0%, Ni: at least 6.0% and at most 8.0%, and N: at most 0.20%.
From yet another standpoint, the present invention is an engine gasket made from a stainless steel having a temper rolled metal structure in which the metal structure is a recovered unrecrystallized structure or a mixed structure of a recovered unrecrystallized structure and a recrystallized structure.
In this manner, according to the present invention, a high hardness with a low C material which could not be obtained with existing technology is made possible, and an improvement in resistance to settlement can be obtained.
In addition, according to the present invention, using a stainless steel having components corresponding to generally known SUS301L, a stainless steel for engine gaskets having high fatigue strength and excellent resistance to settlement and a method for its manufacture are provided.